Many white gold alloy compositions are known. Most are used in making jewelry. There is no standard or specification for the composition of white gold. Many white golds alloys contain nickel or palladium as the bleaching agent. Silver, the whitest of all elements, can be used as the bleaching agent. However, in alloys above 9 karat (i.e., 37.5% gold), silver causes the alloy color to become greenish. Green gold typically includes gold, a high amount of silver, and a small amount of copper.
When nickel is used as the main bleaching agent, the amount of nickel determines the whiteness of the gold. The main advantage of nickel is low cost. However, there are many disadvantages to the use of nickel. Nickel can cause an allergic reaction in approximately 5–10% of the population. If an individual becomes sensitized to nickel, it becomes a lifelong allergy, and skin reactions can be quite severe. Currently, many countries regulate the amount of nickel that can be used in white gold, or require that white gold be nickel-free.
In manufacturing of nickel white gold, there are many disadvantages. If high amounts of nickel (i.e., on the order of 15–30%) are used, the alloy becomes very hard. This makes it more difficult to work the metal, and causes increased stress on the worker. Also, the higher the nickel content, the higher the melting temperatures required. This can cause poor productivity in investment casting, due to gas porosity. Lower nickel content white golds (i.e., on the order of 5–12% Ni) allow for somewhat softer metal and lower melting temperatures, but are grayish in color and typically require a surface plating of rhodium to be commercially acceptable.
Rhodium plating is normally recommended on nickel white gold. Rhodium is a high-reflectivity platinum-group metal, which is extremely hard and corrosion-resistant.
There are no regulations regarding the amount of rhodium applied. Many manufactures use “flash plating”; that is, to apply an extremely thin coating. Such flash-type plating typically wears off with product usage, exposing the underlying white gold. The main reason for flash plating is cost, as rhodium is very expensive. Proper rhodium plating should never wear, due to its hardness. Rhodium should have to be stripped off and plated again when the product is serviced, such as changing a ring's size. The rhodium also provides a barrier between the skin and the nickel-containing white gold.
High nickel white golds generally do not require rhodium plating because of an acceptable white gold color. However, this exposes the skin to higher nickel contents. High-nickel white golds were the general standard in Europe for many years, whereas low-nickel white golds have been the prevalent choice in the United States. This may be why a higher amount of allergic reactions forced regulation in some European countries.
Nickel white gold also can cause a metallurgical problem known as stress corrosion cracking. Typically, many settings on jewelry rings are made from nickel white gold that is die struck from cold-rolled sheet. Stress corrosion cracking is caused by weak grain boundaries that occur in wrought product that is hard and under stress. This is usually seen as broken or cracked prongs on rings. The grain boundaries under stress are corroded easily by many chemicals, including those of a common household variety.
Nickel white golds are also poor in their ability to be reused. This causes a high amount of scrap, which needs to be refined and recycled with the associated costs.
When palladium is used as a bleaching agent for white gold, the amount of palladium in the alloy determines the whiteness of the gold. Typically, palladium alloys are grayish in color. Most palladium alloys still require a rhodium plate for acceptable appearance. The main advantages of palladium white gold are: (1) no known allergic reactions, and (2) relative softness, which allows the metal to be easily worked. However, there are some disadvantages to palladium white golds. Currently, the price of palladium (about $230 per Troy oz.) has caused the cost of materials to rise to such an extent that usage and sales have fallen significantly. The cost of the palladium white gold has at times approached or exceeded the price of platinum, which is regarded as a higher quality product. Palladium prices have dramatic price swings, which create difficulty in consistent pricing at both manufacturing and retail.
High palladium white golds have high melting temperatures, and, consequently, fast solidification times. This limits productivity by limiting the number of units producible by investment casting technique at one pour. Moreover, the high temperature can cause interaction when investment casting with typical investment materials. This causes a sulfur gas to react with the silver (normally high content in the palladium alloys) to form silver sulfide on the product surface, which is very hard to remove.
The cost of refining the palladium in the white gold scrap produced, is expensive, usually between 15% and 25% of the palladium cost.
When silver is used as a bleaching agent, it whitens the gold in 9 karat (i.e., 37.5% gold). In 10 karat, it shows a yellowish tint. In 14 karat, it produces a greenish tint. In 9 karat, the color is a brilliant white, with the silver dominating the color. Silver is the whitest and highest-reflectivity metal known. The higher karat white golds (i.e., 10–18 karat) which use silver must be rhodium plated. However, the plating must be thick to prevent the greenish tint from appearing if the rhodium wears off. The cost of the rhodium plating is also a drawback.
The advantages of silver-bleached white gold are relative softness, which allows easily workable metal, relatively low cost of raw material (about $5 per Troy oz.), low cost of refining and recycling, and absence of known allergic reactions. The disadvantages of silver are the poor color (greenish), and the tendency to tarnish.